The present invention relates to oscillation gyroscopes having PZT (lead (Pb) Zirconate Titanate) thin films.
A conventional tuning-fork type oscillation gyroscope is shown in FIG. 8, while a vibrating reed type gyroscope is shown in FIG. 9. As shown in FIG. 8, the tuning-fork type oscillation gyroscope 21 includes a pair of driving piezoelectric ceramic plates 23, which are arranged on the ends of a connection plate 22. The piezoelectric ceramic plates 23 are parallel to each other and their planes are normal to the X-axis. A detecting piezoelectric ceramic plate 24 is formed integrally with each piezoelectric ceramic plate 23 extending upward from the top center of the ceramic plate 23. The ceramic plates 24 lie in a plane normal to the Y-axis. The X-axis and the Y-axis are perpendicular to each other.
When an alternating voltage is applied to the piezoelectric ceramic plates 23, the piezoelectric ceramic plates 23 oscillate in the direction of X-axis. If torque is applied to the oscillation gyroscope 21 about the Z-axis, the piezoelectric plates 24 distort and produce a voltage corresponding to the distortion. The voltage is detected to obtain the force applied to the piezoelectric plates 24. The force is referred to as Coriolis force Fc and is generally represented by the following formula (1):
xe2x80x83Fc=2mVxc3x97xcexa9xe2x80x83xe2x80x83(1)
In the formula, m indicates the mass of the oscillation gyroscope 21, V indicates the vibrational speed, and xcexa9 indicates the angular velocity of the oscillation gyroscope 21 about the Z-axis. The angular velocity xcexa9 is obtained when the mass m, the vibrational speed V, and the force Fc have been obtained.
As shown in FIG. 9, a vibrating reed type oscillation gyroscope 21 includes a rectangular column-like vibrating reed type vibrator 26, which is made of a constant elasticity metal. A pair of driving piezoelectric ceramic plates 27 (only one shown in the drawing) are adhered to opposing side surfaces, which are separated by 180 degrees, of the vibrator 26. A pair of detecting piezoelectric ceramic plates 28 are adhered to the remaining two side surfaces (only one shown in the drawing). The application of an alternating voltage to the piezoelectric ceramic plates 27 vibrates the vibrating reed type vibrator 26 in the X-axis direction. When torque is applied to the oscillation gyroscope 21 about the Z-axis, the detecting piezoelectric ceramic plates 28 distort and generate a voltage corresponding to the distortion. The voltage is detected to obtain the Coriolis force applied to the piezoelectric ceramic plates 28.
The piezoelectric plates 23, 24, 27, 28 are formed from bulk PZT (lead zirconate/titanate, or ceramics containing a solid solution of lead titanate and lead zirconate). However, it is difficult to form bulk PZT into thin components and thus difficult to reduce the entire size of the oscillation gyroscope.
Additionally, when adhering the piezoelectric ceramic plates 27, 28 to form the oscillation gyroscope like the vibrating reed type oscillation gyroscope 25, the adhering of the piezoelectric ceramic plates 27, 29 adds to the number of manufacturing steps. Furthermore, the adhering accuracy, or the precision of the adhering position is low. This affects the sensitivity of the oscillation gyroscopes 21, 25. Thus, it is difficult to manufacture oscillation gyroscopes having uniform accuracy. If the vibrator has a three-dimensional structure, it is difficult to attach the bulk PZT to an arbitrary location. This restricts the locations where the bulk PZT can be attached.
As apparent from formula (1), an increase in the mass m of the oscillation gyroscope increases the Coriolis force Fc. This increases the distortion amount of the detecting piezoelectric ceramics and increases the detecting voltage of the oscillation gyroscope. In other words, the detecting sensitivity of the oscillation gyroscope increases. It is thus preferred that the oscillation gyroscope have a greater mass to obtain a higher sensitivity. However, if the oscillation gyroscope employs bulk PZT, the mass cannot be increased unless the size of the substrate, which forms the bulk PZT, is increased. Hence, there is a limit to an increase in the sensitivity.
In addition, as apparent from formula (1), an increase in the vibrational speed V increases the Coriolis force Fc. This increases the distortion amount of the detecting piezoelectric ceramics and increases the detecting voltage of the oscillation gyroscope, which in turn, increases the detecting sensitivity of the oscillation gyroscope. However, for example, in the case of a vibrating reed type oscillation gyroscope, if the substrate of the bulk PZT is made thinner, the rigidity of the substrate decreases. Hence, the piezoelectric device is apt to twist and interfere with accurate oscillations. Furthermore, accurate detection is hindered when the detecting piezoelectric device is distorted and twisted.
Accordingly, an oscillation gyroscope 31, which is compact, resists twisting, and increases sensitivity, has been proposed as shown in FIG. 7. The oscillation gyroscope 31 has a rectangular column-like shape and is made of an elastic metal. An opening 34 extends through the lower side (fixed side) of the gyroscope 31, while a perpendicular opening 35 extends through the upper side (free side) of the gyroscope 31. The gyroscope 31 has a first parallel plate portion 32 corresponding to the opening 34 and a second parallel plate portion 33 corresponding to the opening 35. Titanium films are formed on the surfaces of the first and second parallel plate portions 32, 33, while PZT thin films 36 are formed on the titanium films. A plurality of electrodes 37, preferably made of aluminum, are formed on each PZT thin film 36, with a lead wire (not shown) connected directly to each electrode 37. The first parallel plate portion 32 serves as a driving portion for producing vibrations, while the second parallel plate portion 33 serves as a detecting portion. Voltage is applied to the first parallel plate portion 32 to produce vibrations and cause the PZT thin films 36 of the second parallel plate portion 33 to generate a voltage, which is output via the lead wires (not shown) that are connected to the electrodes 37.
However, the connection of the lead wires changes the rigidity of the second parallel plate portion 33. This affects the output voltage generated by the PZT thin films 36. This leads to a shortcoming in which the detecting characteristics cannot be stabilized. The connection of the lead wires also affects the vibrating characteristics of the first parallel plate portion 32. This leads to a shortcoming in which the driving characteristic cannot be stabilized.
Accordingly, it is an objective of the present invention to provide an oscillation gyroscope having a PZT thin film with uniform characteristics.
To achieve the above objective, an oscillation gyroscope according to the present invention comprises a substrate made of an elastic metal and having a substantially rectangular shape, the substrate having first to fourth side surfaces arranged in the circumferential direction, wherein the first side surface and the third side surface, and the second side surface and the fourth side surface are located on opposite sides, the substrate having a fixed end for fixing the oscillation gyroscope, and a free end located on the opposite side of the fixed end, the substrate having a first opening extending from the first side surface to the third side surface at the fixed end side, a first parallel plate portion located on both sides of the first opening and having a pair of side plates including the second and fourth side surfaces, the substrate having a second opening extending from the second side surface to the fourth side surface at the free end side, a second parallel plate portion located on both sides of the second opening and having a pair of side plates including the first and third side surfaces, a titanium base surface formed respectively on the second and fourth side surfaces of the first parallel plate portion and the first and third side surfaces of the second parallel plate portion, a PZT thin film formed on each base surface, an electrode formed on each PZT thin film, and connecting pads electrically connected to either one of the electrodes of the PZT thin films.